One method of extending or adding to the processing capability of a processing unit is to offload some of the work or functions to one or more coprocessors. Typically one processor is a main or control plane processor which is used to coordinate the work of the remaining data plane processors. Each of these processors will typically have their own cache memory. Data plane processors will also typically have their own Local Memory. As is known, caches are essentially temporary storage memory directly or closely linked to a processor. The amount of time required to retrieve information from a cache is significantly less than the time required to retrieve information from hard disk or even from RAM (Random Access Memory. For this reason, a given processor, in a multiprocessor system, typically, when needing information not in its associated cache, will perform a request for the information from memory. Other processors in the multiprocessor system will first perform a lookup or “snoop” request to ascertain if the information being requested is contained in the cache associated with that “another processor.” If so, the information may be directly retrieved from the other cache and each cache in the system is advised if state changes are required in the cache for that memory location, as set forth infra. Thus caches, even of other processors, are used to reduce latency or the time that it takes to retrieve information, especially where that information is likely to be used multiple times by a given processor.
To prevent destruction of data being used or accessed simultaneously by different processors and to further assure that data accessed is the most recently correct value, coherent memory systems have been developed and used to communicate the status of data to all processors using data contained in a given virtual or physical memory location. One example of a coherent memory management system uses a protocol referred to in the industry as a MESI protocol. Each read or write request involves all other caches in the system. After each read or write request relative a given memory location, each cache in the system maintains a record of the cache state with respect to that memory location of Invalid, Shared, Exclusive or Modified in accordance with the MESI protocol. More information on such coherency protocols may be obtained from many sources such as the Internet and so forth. An extended MESI protocol outlined in this invention is set forth in a U.S. Pat. No. 6,334,172 issued Dec. 25, 2001 and assigned to IBM. Known prior coherent memory systems were, however, limited to processors executing the same instruction set, having the same cache hierarchy, and each accessing or addressing memory in the same manner.
A prior art multiprocessing system having heterogeneous processors is shown in an application Ser. No. 09/736,585 filed Dec. 14, 2000, entitled Symmetric Multi-Processing System and assigned to the same assignee as the present invention. This system is also shown in Publication 2002/0078308 published Jun. 20, 2002. In this system, the main processing units (PUs) can access memory at either a physical or a virtual memory location using Load and Store instructions. However, the auxiliary (APU) or synergistic (SPU) processor units access memory using a DMA (Direct Memory Access) unit and have a different cache hierarchy than the PUs. Elsewhere, in IBM prior art, memory management schemes have been described for the DMA units to access memory using a physical and/or a virtual address. In addition, the SPUs have a Local Storage which may be addressed as part of system memory. In other words, the PUs and the APUs were heterogeneous or non-homogeneous.
It may be pointed out that in such heterogeneous processor systems, the APUs are typically specialized processors for doing specific jobs more efficiently, and with less hardware than is used in conjunction with the central or control processors.
Typically, homogeneous multiprocessor systems provide instructions for performing atomic updates of a memory location. Atomic updates are useful in cooperative programming environments to perform operations such as “compare and swap,” “test and set,” “fetch and no-op,” and “fetch and store”. These instructions typically rely on a coherent access of a system memory location. A means for a DMA unit to issue atomic update of memory has been described in IBM prior art. When combined with the present invention, the SPU can perform the above operations in a compatible manner as the control processor(s) and thus participate in a cooperative programming environment with the control processor.
Further in the IBM prior art, methods are described for allowing a DMA operation to be cached and also for data to be predictably pre-fetched. As is known, caches and pre-fetching data typically are provided in a multiprocessor system to improve the performance of a cooperative programming environment. However, to take advantage of these methods in a shared memory system with the control processor, the cached and pre-fetched data must be coherent with the other system caches.
While the characteristics of a prior art non-homogeneous processor can be implemented without a coherent memory structure, the performance and difficulty in programming in such an environment suffers greatly. It would thus be highly desirable to have a system whereby each processor, regardless of processor configuration, can time efficiently and in a coherent manner communicate with other caches used by other processors in the system to minimize time required to retrieve valid and up-to-date information utilized by more than one processor of a multiprocessor system.